If the attachment button is missing, disabled, or your upload is rejected, the usual reason is one of the following:
Your free access is already used up. Chat attachments are available while your one free message is still available, or with Pro access.
The selected model does not support attachments. Switch to a model that supports file and image uploads.
The file type is unsupported. Chatex chat accepts a specific list of document and image formats, not every file type.
You are trying to do a PDF-specific workflow in normal chat. Use PDF Chat when you want grounded question-answering over a PDF.
Video files are not standard chat uploads in the current chat composer. If you only need image inputs, choose a model with attachment support and try again.